Add logfmt selective label extraction

[btaani]

What this PR does / why we need it:
This PR introduces extracting labels from a log line in logfmt, with an extra option to rename them.

For example, this query:

{app=”foo”} | logfmt msg,status

will extract the labels msg and status from the following logfmt line:

level=error ts=2021-02-12T19:18:10.037940878Z caller=client.go:294 component=client host=observability-loki-gateway msg="final error sending batch" status=400 error="server returned HTTP status 400 Bad Request (400): entry with timestamp 2021-02-12 19:18:08.917452 +0000 UTC ignored, reason: 'entry out of order' for stream..."

With the following results:

msg="final error sending batch"
status=”400”

---

Another possible scenario with label renaming:

{app=”foo”} | logfmt message="msg", status

That produces the following results:

message="final error sending batch"
status=”400”

Which issue(s) this PR fixes:
Fixes #3355

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

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[btaani]

/keepalive

[slim-bean]

Oh my, so sorry to let this hit the stalebot 😬

We've talked about adding this for a long time!!

Thanks so much for the PR, we've been super busy lately but will try to take a look at this as soon as we can!!!

[chaudum]

Looks great! Just one question regarding the <label>_extracted labels.

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[btaani]

/hold
I still need to add the benchmark tests

[btaani]

Benchmark test added.
Results:

$ go test -run=NONE -bench='Logfmt' -benchtime 5s -count 5 .
goos: linux
goarch: amd64
pkg: github.com/grafana/loki/pkg/logql/log
cpu: Intel(R) Core(TM) i7-10610U CPU @ 1.80GHz
BenchmarkLogfmtParser-8             	 1919780	      3389 ns/op	     264 B/op	      17 allocs/op
BenchmarkLogfmtParser-8             	 1765072	      3433 ns/op	     264 B/op	      17 allocs/op
BenchmarkLogfmtParser-8             	 1748881	      3428 ns/op	     264 B/op	      17 allocs/op
BenchmarkLogfmtParser-8             	 1749825	      3406 ns/op	     264 B/op	      17 allocs/op
BenchmarkLogfmtParser-8             	 1757325	      3372 ns/op	     264 B/op	      17 allocs/op
BenchmarkLogfmtExpressionParser-8   	  652922	      9622 ns/op	    5411 B/op	      68 allocs/op
BenchmarkLogfmtExpressionParser-8   	  615289	      9584 ns/op	    5411 B/op	      68 allocs/op
BenchmarkLogfmtExpressionParser-8   	  621687	      9592 ns/op	    5411 B/op	      68 allocs/op
BenchmarkLogfmtExpressionParser-8   	  620328	      9693 ns/op	    5412 B/op	      68 allocs/op
BenchmarkLogfmtExpressionParser-8   	  600560	      9659 ns/op	    5411 B/op	      68 allocs/op
PASS
ok  	github.com/grafana/loki/pkg/logql/log	77.919s

[chaudum]

I think we are good to go! 🚢

[owen-d]

This LGTM. Really nice work, apologies it took so long.